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Oil Drop Electric Field Problem

  1. Nov 11, 2014 #1
    1. The problem statement, all variables and given/known data
    An oil drop is charged negatively. How much charge is on the drop if the Electric Field is 6,400 N/C at a distance of 1.2 m?

    2. Relevant equations
    E=k q/r^2
    I don't know if I should be using that equation for this one.

    3. The attempt at a solution
    I tried to do E/(k x r^2) = q but that didn't work
    I also tried E/r^2= q but that also didn't work

    I know the answer has to be 1.3 x 10^3

    Thanks in advance for all the help!
     
  2. jcsd
  3. Nov 11, 2014 #2

    Orodruin

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    You have the correct equation for field strength, you are just not solving correctly for q.

    A good check is whether or not your resulting expression has the correct physical units.
     
  4. Nov 11, 2014 #3
    hmm... okay

    and I'm right in saying k= 9 x 10^9
     
  5. Nov 11, 2014 #4

    Orodruin

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    No. As many other constants, k is dimensionful and k = 9 x 10^9 N m^2/C^2 - units are important and the units of your final answer must also make sense.
     
  6. Nov 11, 2014 #5
    Oh I have to square root k x r^2 !
     
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